1. Field of the Invention
This invention relates to an amplification circuit and to a technique which will be effective when utilized for an amplification output circuit for a CCD (Charge Coupled Device), for example.
2. Description of the Prior Art
A circuit such as shown in FIG. 6 of the accompanying drawings has been used as an amplification circuit for a CCD. A signal charge is transferred by the CCD to an output diffusion layer (which is expressed structurally equivalently in the form of a diode D), and is converted to the form of a signal voltage by a capacitor C1 consisting of its junction capacitance. This signal voltage is outputted through a source-follower circuit consisting of an amplification MOSFET Q2 and a load MOSFET Q3. Such an amplification circuit is referred to as an FDA (Floating Diffusion Amplifier). The MOSFET Q1 has the signal charge retained in the capacitor C1 reset by a reset pulse .phi.R at a reference voltage VR when the signal voltage corresponding to the signal charge is amplified and outputted or in other words, before a next signal charge is transferred.
Such an FDA is described, for example, in "CCD Camera Technique", page 64, published by Radio Kijutsusha, Nov. 3, 1986.
The sensitivity of the FDA described above is given by the product (As/Cl) of the capacitance value of the capacitor C1 inclusive of the parasitic capacitance in the reset MOSFET Q1 and in the amplification MOSFET Q2 and the gain (As&lt;1) of the source-follower circuit. The improvement of the gain As is limited by the relation As&lt;1. Therefore, the improvement of the sensitivity of the FDA depends on how much the capacitance value of the capacitor Cl can be reduced. In the conventional FDAS, therefore, great efforts have been made to miniaturize the diode D1 and the amplification MOSFET in order to reduce the capacitance value of the capacitor C1. If the size of the amplification MOSFET Q2 is reduced, however, the output current essentially becomes smaller, and there occurs the contradictory problem that the load driving capacity of a post stage circuit is lost. Therefore, it has been customary to cascade a plurality of source-follower circuits so as to make up for insufficiency of the driving capacity of the initial stage circuit.
In the FDAs available at present, the capacitance value of the capacitor Cl described above has come to below 10.sup.-14 F and the voltage sensitivity has exceeded 10 microvolt per electron. From the aspect of application, however, the number of electrons handled per signal is at most some dozens and a further increase of the output signal amplitude has been desired. The real sensitivity is determined by an S/N (signal-to-noise ratio) and in order to improve this S/N, reduction of thermal noise that occurs at random is particularly indispensable.
The thermal noise occurring in the CCD itself has been reduced remarkably and the thermal noise in a CCD imaging device is determined by the thermal noise generated in the FDA. The principal components of the noise in the FDA are the reset noise of the capacitor C1 and the 1/f noise of the amplification MOSFET Q2. The reset noise is proportional to the square of the capacitor Cl, and decreases with the reduction of its size, whereas the 1/f noise is substantially in inverse proportion to the reduction of the size, on the contrary.
Since such a random noise is contained in both of the dark output (reset voltage) and the bright output (signal charge output), it can be offset by obtaining the difference by a correlational double sampling (CDS) circuit and generating a signal. However, there is a limit to the noise reduction by the CDS circuit because the distortion of waveforms up to the CDS circuit and the disturbance of the waveforms due to various jump in pulses resulting from the extension of wirings, and so forth, exist.
It is therefore an object of the present invention to provide an amplification circuit which can accomplish a high sensitivity with a simple circuit structure.
It is another object of the present invention to provide a high sensitivity amplification circuit which will be suitable for a CCD.
These and other objects and novel features of the present invention will become more apparent from the following description of this specification when taken in conjunction with the accompanying drawings.